Plasmon-Resonant Gold Nanostars with Variable Size as Contrast Agents for Imaging Applications

Plasmon-resonant nanostars (NSts) have recently found applications in biophotonics due to their unique optical and chemical characteristics, showing comparable or superior properties than other anisotropic plasmon-resonant nanoparticles. In this paper, we synthesized gold NSts by the seed-mediated surfactant-free method. By varying the diameters and amount of spherical seeds we tuned the final NSts tip-to-tip sizes to 50, 82, 100, and 120 nm ensuring the plasmon-resonant peak location between 710 and 830 nm, and the scattering/absorption ratio at the plasmon-resonant wavelengths being 0.12, 0.25, 0.30, 0.35 correspondingly. We investigated the application of the NSts as contrast agents for imaging techniques operating at visible and infrared wavelengths: optical coherence tomography (OCT) and Doppler OCT with the spectrum centered at 930-nm wavelength, as well as for conventional confocal laser scanning microscopy (CLSM) working at 633 nm. The most intense OCT signal was registered from the largest NSts, in correspondence with spectroscopy measurements at 930-nm wavelength. For imaging of nanoparticles incubated with living cells, we applied CLSM in combined scattering and transmission modes, and observed localization of the NSts on the cell surface. Due to the highest scattering at the CLSM operating wavelength, the strongest signal was obtained from the 82-nm particles; the lowest intensity of the CLSM backscattered signal was detected from the cells labeled with the smallest NSts. Thus, by tuning the initial concentration of seeds, it is possible to adjust the size (and scattering properties) of the nanostars to the operating wavelength of the optical device to achieve the best performance.